Behaviour
Our first module is the most
fundamental: behaviour. The aim of systems neuroscience is to figure out
the biological basis of thought and behaviour. This is done by
correlating biology -- notably, measuring or manipulating neural firing
rates -- with behaviour. That's how it is done even if we are more
interested internal reasoning, emotion, or motivation than in behaviour
itself: since we cannot yet directly measure thoughts, the human subject
or experimental animal's actions are always used as a proxy.
For a cognitive feature of interest, choosing the right behaviour to
train or measure is no simple task. To yield insightful experiments, the
behaviour must be the right combination of robust, naturalistic, and
truly reflective of the relevant thought process.
In this module, we wanted to find out to what degree rats are capable of
recognizing and manipulating objects. We know that rats excel in a
foraging task, in which they learn to navigate to a moving spot of light
in order to earn a sugary reward. This indicates that they are capable
of locating and navigating to sensory 'objects'. This, however, is a
very limited component of the kinds of interactions humans have with
objects, which culminates in the use and invention of complex tools.
Taking one step toward this extreme, we would like to know whether rats
can learn not just to navigate to an object but also to move it to a
target location. Specifically, we attempted to train our rats to push a
marble into a hole. Note that this is more complex than, say, pushing a
lever to earn a reward; pushing a lever simply requires a particular
motor act when the rat is in front of the lever. Getting a marble into a
hole, however, requires a distinct set of pushes depending on the
location of the marble and hole. This is a more flexible type of
behaviour; it requires an abstract understanding that the marble should
fall in the hole and that the rat is capable of causing that to happen.
Can rats easily learn this kind of object-oriented thinking? Can we
design a behavioural assay to effectively test that question? If yes to
both, can we look in the brain and discover the neurobiology behind this
simple model of object recognition and manipulation?
With this tricky task, nuanced question, two rats and one week of
training, we were doomed to inconclusivity. Nonetheless, here are our
methods, data, and conclusions:
Materials & Methods
We trained our rat in 13 sessions of 30-50
minutes over 9 days, in a 46 x 80 cm behaviour rig.
The floor consists of two stacked rectangular acrylic boards: one with a
3-cm hole in the center to fit the 2.6-cm marble, and the other with
larger central hole of 20-cm diameter. Stacking these allowed us to
place three different 3D-printed circular sections in the middle
(schematic below). One was otherwise flat (most difficult); one was
sloped so that the marble would slide into the hole if it came within 10
cm of the center (medium-difficulty); and one was sloped across the
entire 20-cm diameter (least difficult). To make it easier for the
marble to roll into instead of past the hole, we sanded the board to add
friction, and installed grooves along diameter-sections of the
most-sloped centerpiece.
Left: Schematic of the flat, most
difficult (top), partially ramped, medium-difficulty (middle), and
fully ramped, least difficult (bottom) centerpieces, used in training.
Right: 3D rendering of the medium-difficulty centerpiece
The reward port contained an infrared beam and detector and a
strawberry-Ribena juice dispenser. If the rat sticks its head into the
reward area at an appropriate moment, the IR beam breaks and then a drop
of juice is dispensed. The marble detector similarly contained an IR
beam that, when broken by a marble falling through the hole, would send
a signal to the computer. Each sessions was filmed with an overhead IR
camera. We coded all training automation and time-stamp-saving in
Bonsai. Video
analysis was performed in MATLAB.
Behavioural training progressed through several stages. First, the rat
habituated to the behaviour rig and human handling over 2 days. Next,
over 6 days the rat was trained on an association between a beeping
sound (on for 50 seconds or until the rat arrived at the reward port,
and then off for 10-30 seconds; as training progressed the beeping was
reduced to 20 seconds on) and a juice reward in the reward port. During
this training, when the rat pokes its head in the reward port, it will
receive juice, if and only if the beeping sound is on at that moment.
After entraining this beep-juice association, we put the marbles in the
arena.
The arena was now set up with 5-7 marbles placed around the outer
circumference of the centerpiece at the beginning of each session. The
plan at this point was as follows: reward any interaction with marbles
by turning on the beeping and reward availability; once the rat starts
to regularly interact with marbles, decrease this manual rewarding and
only provide reward (automatically) when a marble falls through the hole
(of the least difficult centerpiece); once the rat starts to regularly
push marbles into this sloped centerpiece, switch to the
medium-difficulty centerpiece; and once the rat comes to regularly push
marbles into the medium-difficulty hole, switch to the flat centerpiece.
Finally, once the rat comes to regularly push marbles into the hole,
modify the parameters of the task to test whether the rat has truly
learned the abstract, object-oriented thinking we attempted to entrain
or if it instead learned a simpler rule
with which it just happened to successfully get marbles in the hole. For
example, we could place marbles in another arena adjoined to the one
with the hole by a narrow passage. It is unlikely for the rat to push
marbles across the narrow passage unless it truly possessed the goal of
getting marbles into the hole.
Given just one rat and 3 days of training with marbles, this training
protocol did not yield a definitive result. As shown below, we instead
were able to make some observations about our rat's particular habits
during training. We modified our reward protocol accordingly, now
providing chocolate treats by hand whenever the rat interacted with the
marble or pushed it into the hole. This more effectively encouraged the
rat to interact with the marble.
Results
Days
1-6 (sessions 1 - 8) -- Beep-Juice-Association Training:
Over these sessions, the rat was trained on the above-described
beep-juice-association training protocol. While managing to stay awake,
the rat only sporadically visited the reward port. After some manual
intervention -- dropping a trail of juice leading to the reward port --
its interest in the port marginally increased. As this point, we
increased the inter-beep-interval (up to 30 sec) and decreased the
beeping duration (down to 20 sec). However, a robust
beeping-followed-by-running-to-collect-juice association never fully
formed.
Video and heat map showing the rat's
position over beep-juice-association training session #5. While
showing some interest in the reward port (bottom-middle region), the
rat spent more time in a corner (top-left region) and some time in the
middle (middle region).
Days
7 - 9 (sessions 9 - 13) -- Marble-Pushing Training:
Sessions 9-11 followed our original training protocol from Materials and
Methods. However, during these sessions the rat demonstrated a
surprising behavioural pattern that led us to modify our reward protocol
in sessions 12-13. At the beginning sessions 9 - 11, and sporadically
throughout the session, the rat demonstrated a high intrinsic interest
in the marbles. It interacted with them, mostly ignoring the beeping and
potential juice reward that followed. Indeed, with the least-difficult
centerpiece in place, the rat quickly pushed several marbles into the
hole, and then ignored the resulting reward availability at the port
(video below). This demonstrated an intrinsic interest in interacting
with the marbles, which could potentially be shaped toward specifically
pushing them into the hole. It also showed, however, that our beep-juice
association was inadequate for entraining this task.
In sessions 12-13, we modified our reward
protocol accordingly, now providing chocolate treats by (Lory's) hand
whenever the rat interacted with the marble or pushed it into the hole.
As this required no effort on the rat's part beyond gulping up a
chocolate lowered to its mouth, it now collected its reward every time
the reward was earned. During session 12, the medium-difficulty
centerpiece was in place, and the rat was rewarded each time it pushed a
marble even slightly -- however, as the session progressed only pushed
toward the center of the arena were rewarded. During this session, the
rat pushed 6 out of 7 marbles into the hole. In addition, when it pushed
marbles away from the center, it started to demonstrate a new behaviour:
stopping and looking upward, perhaps in anticipation of Lory's
sugar-bestowing hand (video below). During session 13, the most
difficult centerpiece was in place, and the rat was only rewarded for
pushing a marble toward the center. It pushed 4 out of 5 marbles into
the hole, and demonstrated a sustained interest in the marbles.
Video: The rat had an untrained interest
in the marbles but lacked interest in the resulting juice
availability.
Video 1: After having learned to
interact with marbles in order to receive a treat, we started to only
reward pushes toward the center. At this point, the rat paused and
looked upward after touching a marble or pushing it away from the
hole, presumably in expectation of chocolate; Video 2: An example of
the rat pushing the marble toward the center in order to earn a
chocolate treat -- the culmination
of our training.
Conclusions
We were interested in finding out to what degree rats can learn
abstract, object-oriented tasks. The task we selected was to push
marbles placed around an arena into a small hole at the center of the
arena. Previous work has shown that rats can learn conceptually similar
and physically even more difficult tasks, such as basketball (video
below). Our goal was to optimize a training protocol for a simpler task;
once learned, our marble-pushing task could then be systematically
modified and extended in order to probe exactly what types of abstract
object-manipulation tasks rats are capable of. We did not get that far
after 8 days of training. However, we did make one key update to our
original materials and methods -- replacing the 6 days of inefficient
juice-beep-association training with a chocolate reward as soon as the
rat pushes the marble. With this change, our rat began to learn to push
marbles. We believe that some proportion of rats trained on this
protocol would learn to push marbles into a hole, and could then present
a rodent model system for studying the cognition and neurobiology of
object manipulation.
Rats are capable of basic
object-manipulation tasks -- can we harness that as a model system in
the laboratory?
